Higher plants are autotrophic organisms that can synthesize all of their molecular components from inorganic nutrients obtained from the local environment. Nitrogen is a key element in many compounds present in plant cells. It is found in the nucleoside phosphates and amino acids that form the building blocks of nucleic acids and proteins, respectively. Availability of nitrogen for crop plants is an important limiting factor in agricultural production, and the importance of nitrogen is demonstrated by the fact that only oxygen, carbon, and hydrogen are more abundant in higher plant cells. Nitrogen present in the form of ammonia or nitrate is readily absorbed and assimilated by higher plants.
Nitrate is the principal source of nitrogen that is available to higher plants under normal field conditions. Thus, the nitrate assimilation pathway is the major point of entry of inorganic nitrogen into organic compounds (Hewitt et al. (1976) Plant Biochemistry, pp 633-6812, Bonner, and Varner, eds. Academic Press, NY). Although nitrate is generally the major form of nitrogen available to plants, some plants directly utilize ammonia, under certain conditions.
In Saccharomyces cerevisiae, the transport of ammonium across the plasma membrane for use as a nitrogen source is mediated by at least two functionally distinct transport systems. Expression of an Arabidopsis cDNA in a mutant yeast strain deficient in two ammonium uptake systems allowed the identification of a plant ammonium transporter. The isolated cDNA encodes a highly hydrophobic protein with 9-12 putative membrane spanning regions. Sequence homologies to genes of bacterial and animal origin indicated that this type of transporter is conserved over a broad range of organisms suggesting that this gene encodes a high-affinity ammonium transporter (Ninneman et al. (1994) EMBO J. 13:3464-3471). A gene encoding an ammonium transporter has been identified in yeast which is most highly expressed when the cells are grown on low concentrations of ammonium or on ‘poor’ nitrogen sources like urea or proline. This gene is down-regulated when the concentration of ammonium is high or when other ‘good’ nitrogen sources like glutamine or asparagine are supplied in the culture medium. The main function of this gene appears to be to enable cells grown under nitrogen-limiting conditions to incorporate ammonium present at relatively low concentrations in the growth medium (Marine et al. (1994) EMBO J. 13:3456-3463).
Genes encoding high affinity ammonium transporters have yet to be identified in corn, soybean and wheat, although ESTs encoding peptides with similarities to cDNAs encoding high-affinity ammonium transporters are found in the NCBI database. Rice ESTs having General Identifier Nos. 568344 and 2309655 encode peptides with similarities to high-affinity ammonium transporters. Genes encoding ammonium transporters have yet to be identified in corn, rice, soybean and wheat, although an EST encoding a peptide with similarities to cDNAs encoding ammonium transporters is found in the NCBI database having NCBI General Identifier No. 5005512.